1. Technical Field
This invention relates generally to apparatus used in the primary cementing of wells. More specifically, the invention relates to stage collars used in multistage cementing operations.
2. Discussion of Related Art
During well drilling operations, particularly in areas such as the North Sea, gas sands and other weak or low pressure zones are frequently penetrated at shallow depths less than, for example, 2000 feet below sea level. These sand pockets or lenses tend to be randomly distributed and are difficult to detect except by drilling and wireline logging.
Due to their small size and low pressure, the energy in shallow gas pockets is relatively low but porosity and permeability can be high. Furthermore, the primary hydrostatic pressure control means, such as a mud column used to contain the gas lens pressure during drilling, is relatively low. If the primary hydrostatic control is lost, the result can be short-duration but violent gas flow, blowouts and/or undesirable and sudden platform setting.
Stage cementing is a technique which can be used to control and confine the shallow gas formations during and after cementing operations performed through a weak zone. The top of the first cementing stage is located above the weak zone. When it has been determined that the first stage has successfully sealed off the weak zone, the second stage can be completed. The resulting cement sheath which thus surrounds the well casing string replaces the drilled-out natural barriers and thus prevents vertical flow.
Stage collars are used in stage cementing to solve the problem of excessive cement hydrostatic pressure. Cementing hydrostatic pressure becomes excessive when a column of liquid cement exerts a pressure that exceeds the formation gradient. However, as cement sets, it will support its own weight and will not transmit the hydrostatic load of a liquid cement column above. Therefore, by building a column of cement in stages of a set cement column and liquid cement, the overall hydrostatic pressure at a given point in time is reduced.
Known stage collars also solve other problems associated with primary cementing of well casing by permitting the cement to be pumped through the drill pipe. When the cement is pumped through the drill pipe, the time and quantity of fluid needed to displace the cement are greatly reduced. Also, when it is necessary or desirable to cement to the surface, which is often done at shallow sites, the drill pipe cementing technique reduces cement waste to the volume of the drill pipe. Contamination is also reduced.
The known stage collars, however, have numerous drawbacks and undesirable features. Stage collars which are not drill pipe actuated require a drill-out procedure for the plugs, darts, seats, and other hardware. Many of the known collars require more than one run down the bore hole to perform a two-stage cementing operation. This greatly increases the time and cost required to complete a stage cementing operation.
Another problem with the known collars is that the closed collars can be accidentally reopened after the stage cementing operation is completed. Also, the stage collars so not adequately isolate the casing interior from the drill pipe, thus requiring the use of a well head closure device. Although drill pipe-actuated stage collars are known, such as disclosed in U.S. Pat. No. 3,768,562 issued to Baker, the collar does not have a uniform bore after removal of the drill pipe and actuating tool, and the collar is not locked closed. Furthermore, this known device is not a positive seal stab-in type design, and relies on sliding seal cups or isolation packers, which can wear down.
An apparatus for performing a two-stage cementing operation with one run down the hole is known; however, this apparatus requires drill-out to achieve a uniform bore. This drill-out procedure is an additional and costly step, and can damage the stage collar and reduce its ability to isolate the weak zone. This apparatus also requires the use of known length-compensating subs (bumper subs or slip joints) and associated tools. Also, the associated stage collar is not drill pipe actuated but, rather, is hydraulically actuated open and closed using plugs and darts.